FACTOIDS Each week, a shark eats about 1 to 10 percent of its total body weight

Hammerhead Sharks

ORDER: CARCHARHINIFORMES.

Class: Elasmobranchii (sharks and rays)
SCIENTIFIC NAME: SPHYRNA MOKARRAN.
Common name: Hammerhead, bonnethead, scoophead shark

DISCRIPTIVE FEATURES: Posesses a flat hydrofoil head. Also has a long dorsal fin. Unlike the scalloped Hammerhead shark, the great hammerhead only has 2 bumps on it's snout.
HABITAT: Resides in both coastal and pelagic waters and is semi-pelagic. It's a nomadic shark and is also migratory.Has been known to enter water as shallow as 1 meter.
FEEDING HABITS: It swims along the sea floor swinging it's head from side to side in order to detect it's prey.
REPRODUCTION: Viviparous.The yolk-sac placenta being inextricably interdigitated with the uterine wall while in some (but not in all) species the umbilical cord possesses numerous finger-like projections believed to have a respiratory function.
SIZE: Can attain a maximum length of anywhere between 5.5 to 6.1 meters (from 10 to over 20 feet).

Remark: Marine, coastal; occasionally in brackish water. Distribution: global (chiefly warm waters). Head laterally expanded, with eyes and nasal openings much widely set than in other sharks. No spiracle. A maximum length of 4.5 m was reported for @Sphyrna tudes@@. They feed on a wide variety of bony fish, elasmobranchs, cephalopods, crustaceans, and other prey.

GENERAL DESCRIPTION
The hammerhead shark has a wide, thick head with the eyes at the margins. The head is indented at the center of the "hammer," which is almost rectangular in shape. This shark is gray-brown above with an off-white belly. The first dorsal fin (the large fin on the top of the shark that most people associate with sharks) is very large and pointed.

TEETH
Teeth are triangular with extremely serrated edges.

There are 9 species of hammerhead sharks, ranging in size from about 3 feet (0.9 m) long to over 20 feet (6 m).

DIET AND FEEDING HABITS
The hammerhead is a fierce predator with a good sense of smell that helps it find its prey. The hammerhead eats fish, including rays, and other sharks, squid, octopuses, and crustaceans. The hammerhead has been known to be cannibalistic.

Stingrays seem to be a particular favorite of the hammerhead. It kills the ray by using its "hammer" to pin the stingray down while it takes bites from the ray's wings.

HAMMERHEAD SHARK ATTACKS
Many of the hammerheads are harmless to people, but a few species, like the hammerhead, can be very dangerous.

HABITAT
The hammerhead swims in warm and relatively warm water along the coastlines. They live over the continental shelves and the adjacent drop-off (the upper part of the mesopelagic zone) to depths of about 260 feet (80 m).

DISTRIBUTION
The hammerhead is found in tropical and sub-tropical waters worldwide.

MIGRATION
The hammerhead migrates seasonally to cooler waters during the summer.

HAMMERHEAD SHARK CLASSIFICATION

Kingdom Animalia (animals)
Phylum Chordata
SubPhylum Vertebrata (vertebrates)
Class Chondrichthyes (cartilaginous fish)
Subclass Elasmobranchii (sharks and rays)
Order Carcharhiniformes
Family Sphyrnidae

Genus Eusphyra

Species blochii: the winghead shark

Genus Sphyrna

Species corona: Scalloped bonnethead

Species couardi: Whitefin hammerhead

Species lewini: Scalloped hammerhead, bronze hammerhead, kidney-headed shark

Species media: Scoophead

Species mokarran: hammerhead

Species tiburo: Bonnethead, shovelhead, bonnet shark

Species tudes: Smalleye hammerhead

Species zygaena: Smooth hammerhead, balance fish, black hammerhead

Great hammerhead shark

Sharks are chondrichthyes, a type of fish that also includes skates and rays and first appeared on Earth nearly a half-billion years ago. Chondrichthyes lack bone and have a skeleton made entirely of cartilage, the same flexible material in human ears and noses. There are eight orders and nearly 400 different species of sharks.

The shark species have widely varying diets. Some sharks concentrate mainly on certain types of prey, such as crustaceans or small fish. The whale shark feeds on microscopic zooplankton and small fish. Others are opportunistic, meaning they will feed on whatever edible material or creatures are available.

While sharks are among the most feared of creatures, most species have never attacked humans. Scientists now believe that attacks most often occur when sharks encounter swimmers or divers and mistake them for their usual food — sea lions, turtles or large fish. They usually release the victim after realizing their error, and inflict only minor injuries. Much rarer — and deadlier — are "bump and run" and "sneak" attacks in which the shark deliberately circles and strikes, inflicting multiple bites.

According to the Florida Museum of Natural History, three varieties of sharks pose the greatest threat of a potentially lethal attack: the great white shark (Carcharodon carcharias) — immortalized in the movie Jaws; the tiger shark (Galeocerdo cuvier); and the bull shark (Carcharhinus leucas). Other species have also been known to attack more infrequently, such as the great hammerhead (Sphyrna mokarran) and the oceanic whitetip (Carcharhinus longimanus), which is believed to have devoured sailors after the sinking of the U.S.S. Indianapolis during WWII.

An estimated 100 million sharks of various sorts are killed each year, worldwide. Great numbers of sharks are accidentally killed en masse by fishermen's nets. In other instances, they're deliberately hunted down, de-finned, and tossed back in the water to die.

Sharks aren't just misunderstood, they're hardly understood at all. So little is known about the most infamous of the lot, the great white, that we don't even know if they're endangered. There has yet to be an accurate worldwide count. It is assumed they give birth to live pups (as opposed to laying eggs), as do most large sharks, but scientists still have no proof. Most researchers agree that while great whites are deadly predators, attacks on humans are usually a blunder on the shark's part. For unknown reasons (possibly bad eyesight), the shark mistakes the victim for a large sea mammal like a sea lion.

Some things we do know: Sharks sit at the top of the ocean's food chain, which means most of the more than 350 species have few natural predators except humans. Sharks don't have bones. Instead their internal frames are made of cartilage. Sharks have what some call a sixth sense. They can sense electrical and magnetic fields, which helps them detect prey and orient themselves using the earth's magnetic fields. Sharks' diets vary widely. The largest of the bunch, the whale shark, feeds mostly on plankton, while tiger sharks will eat most anything (see above list). Most species grow slowly and mature late. It can take up to 20 years for some species to reach reproductive age. This means that population depletion can be especially devastating.

Dr. Gruber is working to fill in some of the blanks in our knowledge about sharks. By studying lemon sharks in the Caribbean he hopes to begin answering some basic questions like: Where and how far do they range? What is their growth and life cycle? And how do they reproduce? I'm going to take you along with me into Doc Gruber's world, where we'll experience these frightening, elegant, puzzling creatures together. Hopefully, by the time we leave, some of us will be able to look a shark in the eye without obsessing on what his teeth might feel like.

Miscellaneous Shark Facts A Few Shark Facts

1.) Like many animals, sharks have their own special behavior to warn other animals away. They arch their backs and lower their fins.
2.) On average, more people in the United States are killed by honeybees than by sharks worldwide.
3.) One hundred million years ago, sharks comprised 60 percent of all the ocean species; they are just three percent of the ocean's species today.
4.) In 25 years, a Grey shark can produce up to 20,000 teeth - which is why shark teeth are the world's most commonly collected animal artifact.
5.)Certain shark species bite with the force of 42,000 pounds per square inch - compared to the adult male human's bite of 150 pounds per square inch
6.)Sharks have low "fecundity", which makes it harder for their populations to recover quickly when they're over-fished.
7.)Of the 350 known shark species, only a small percentage pose a hazard to humans.
8.)Every year, 1,500 to 2,000 sharks are found trapped and usually dead in the shark nets protecting the South African coast.
9.)At least eight known species of shark are endothermic, or warm blooded - controlling their temperature much as humans do.
10) Sharks can tan, but rarely get cancer. Is it possible that the cure for cancer lies within a shark?

FACTOIDS

Each week, a shark eats about 1 to 10 percent of its total body weight.
Most sharks have very powerful jaws.
All sharks will eat weak, sick or injured animals, including other sharks.
Using special sensory organs known as ampullae of Lorenzini, sharks can detect the weak electrical fields produced by all living animals.
Most sharks would rather avoid human contact; and some, like the huge whale shark, are strictly vegetarian.
More people die in 10 minutes on the roads than in a whole year from shark attacks in the water.
Sharks do not have a swim bladder, instead many of them have large livers to provide buoyancy.
Only 32 shark species of the approximately 400 identifies species have been known to attack humans

ANCIENT HAMMERHEADS

Paleocarcharias
Another shark from the teeming waters of Germany 150 million years ago, Paleocarcharias was a large primitive relative of modern bull sharks and hammerheads.

SPECIFIC CHARACTERISTICS

Dorsal Fins Located at the top of the shark, the dorsal fin and the smaller secondary dorsal fin are used for stability while swimming.
Pectoral Fins These fins are usually located near the front of the shark. Not unlike that of an airplane, the pectoral fins are used to provide lift.

Eyes
Unlike those of other fishes, a shark's pupil can dilate and contract. The eye has a layer of reflecting plates called the tapetum lacidum behind the retina. These plates act as mirrors to reflect light back through the retina a second time. The tapetum lucidum of a shark is twice as effective as that of a cat. Sharks have a laterally compressed, basic vertebrate eye with large spherical lenses.
Cone cells are present, indicating that sharks may have some sort of color vision. Some species have an eyelid like structure called a nictitating membrane. The nictitating membrane protects the eye from being injured by thrashing prey while the shark is feeding. Eye size and position vary, depending on the particular habitat or behavior of the species. In general, deep-water sharks have bigger eyes than shallow-water sharks.

Sense Of Sight
In Clear water, a shark can see at a distance of up to about 15 meters (50 ft.). Although little is known of a shark's visual abilities, it appears as if they can see well in dim light. Their eyes are particularly sensitive to moving objects and have many rods, which aid in seeing contrasts very well.

Shark Teeth - Mouth
Certain shark species bite with the force of 42,000 pounds per square inch - compared to the adult male human's bite of 150 pounds per square inch.Some species of sharks may shed as many as 30,000 teeth in a lifetime. The mouth is usually ventral and may have labial folds or furrows.
Teeth are modified, enlarged placoid scales. Sharks have many rows of teeth attached at their bases by connective tissue. Several rows of new teeth continually develop behind the outer rows of functional teeth. As older, damaged or worn teeth fall out, new teeth move in from back to front to take their place.

Caudal Fin
The upper lobe of a shark's caudal fin is larger than the lower lobe. As the caudal fin moves from side to side to thrust the shark forward, it also moves upward. As the caudal fin continues to lift, the shark's head points down. Sense the upper lobe is larger, the result is a forward and down motion. Shape and size of the caudal fin can vary from different species

Heart
A shark's heart is a two-chambered S-shaped tube, small in proportion to body size. Blood flows from the heart to the gills and then to body tissues.

Circulatory System
Fast-swimming sharks, such as great whites and makos, have a body temperature that can be quite a bit higher than the surrounding water (up to 8*C or 14.4*F higher). This heat is due to the modified circulatory system associated with the red muscle.
As red muscle functions, it generates heat. Muscle-generated heat warms the blood circulating through the red muscle, which then travels back to the heart through veins. Thus, blood returning to the heart from the muscle is warmer than blood traveling from the heart to the muscle.
Due to the nearness of arteries and veins, heat passes from warmer veins to cooler arteries within the shark's body, rather than dissipating to the cooler environment. This modified circulatory system retains heat in the red muscle.
Sharks have low blood pressure. The walls of the pericardium (the membranous sac that encloses the heart) are rigid, creating a suction within the pericardium to maintain the flow of blood. To circulate blood throughout their bodies, many sharks (but not all) must swim continuously

Skeleton
A sharks skeleton is made of cartilage and may be partially calcified with calcium phosphates and carbonates, especially in the vertebral column. This calcified cartilage is not true bone.
Shark cartilage contains an active ingredient that has been known to inhibit tumor growth. Sharks rarely develop cancer. A shark's cranium is a single compact cartilaginous block which encloses the brain, olfactory, and auditory capsules. Jaws are loosely attached to the cranium. Vestigial ribs give no support.

Liver
A shark's liver is made of two large lobes that concentrate and store oils and fatty acids. The livers functions are - energy storage and buoyancy.
A shark's liver makes up 5% to 25% of its total body weight.

Hammerhead Video

(click on the link to see how a hammerhead shark moves)

pnm://video.discovery.com/DCOL/sharkweek/hammer28.rm

SHARK ATTACK

Sharks seem slow and plodding as they quietly cruise coastal shorelines but some are known for their stunning speed and brutal attacks.